In continuous casting, the molten steel contained in the casting ladle, where its composition has been adjusted, does not flow directly into the bottomless, cooled wall mould(s) where it begins to solidify. First it passes through a container called a "tundish", with an inner refractory lining, which has a number of functions. First, the bottom of the tundish contains one or generally several orifices called "nozzles", each overhanging a mould, which enables it to distribute the molten metal into the moulds, while the casting ladle has only one orifice through which the metal flows. In addition, the tundish provides a reserve of metal which makes it possible, when a ladle becomes empty, to continue casting the metal while the empty ladle is removed and a new ladle is installed and opened. In this way it is possible to cast several successive ladles without interruption (this is known as "sequence casting"). Finally, the tundish provides an advantageous site for undesirable non-metallic inclusions present in the molten steel to settle out, the benefit increasing the longer the metal remains in the tundish.
Some continuous casting installations include a facility for acting on the temperature of the molten steel using a heating device. This action can make it possible:
to decrease the amplitude of the variations in temperature of the molten steel leaving the tundish during casting: a ladle generally takes several tens of minutes to empty and during this period the molten steel that it contains can lose several tens of degrees; heat input to the tundish, particularly at the end of casting, makes it possible to compensate at least partly for these losses of heat, so as to limit the variations in temperature of the metal leaving the tundish within a range of a few degrees throughout the casting period; PA1 to reduce the required temperature of the metal during its earlier treatment stages, thereby increasing the productivity of the foundry (the periods of heating the metal during treatment in the converter, the electric furnace or the ladle furnace can be reduced) and achieving savings on the consumption of the refractory materials lining the various containers.
In general, this increased control of the temperature makes it easier to obtain a temperature of the steel in the tundish relatively close to the liquidus temperature of the casting grade. The difference between these two temperatures is called "superheat". From a metallurgisal point of view, a small temperature difference helps to obtain a solidified product that in section has little segregation of alloy elements such as carbon, manganese and sulphur, thereby achieving good homogeneity of its mechanical properties. This advantage is particularly important when casting grades of steel with high alloy element content. In addition, a small temperature difference enables the product solidification time to be reduced: this makes it possible to cast the product at a higher speed, leading to increased productivity of the foundry, or to construct a relatively continuous casting machine, thereby reducing the investment.
A first method of supplying heat to the metal passing through the tundish consists in making at least part of said metal flow inside a channel surrounded by a coil with appropriate characteristics, the currents induced in the metal causing it to be heated by the Joule effect. This solution is relatively expensive and the size of the coil makes it difficult to apply to small installations, or installations not initially designed to be so equipped.
Another solution consists in installing above the metal in the tundish one or even several plasma torches. Document WO 95/32069 in particular describes a tundish so equipped. It will be recalled that the operating principle of a plasma torch consists in blowing onto the material to be heated a gas under pressure (plasma gas), such as nitrogen or argon, through which an electric arc created between a cathode and an anode is passed. The gas is thus partially ionised and is heated to a very high temperature (4000 to 15000 K). It possesses very high thermal conductivity and radiation power which make it suitable for performing fast, intense heat exchanges with the material to be heated. By varying the pressure of the gas and the intensity of the current, it is easy to obtain the powers of several hundred kW needed for heating the steel in the tundish using a torch that is small enough to be installed even on a small tundish. Two designs of torch are used for this application. In "injection" plasma torches the cathode and anode are both incorporated in the torch. In "transferred arc" plasma torches, only the cathode is incorporated in the torch, the anode being formed by the molten metal to be heated. For this purpose, the base of the tundish encloses an electrically conducting element which is brought into contact with the molten metal during casting and connected to the positive terminal of the electric power supply of the torch. It is also possible to use the reverse polarities to those previously described.
The area of the tundish in which the torch is located must be covered by a cover with a refractory lining. This cover, under which an inert gas such as argon can be blown in addition to the plasma gas (or to replace it during periods when the torch is not in use), makes it possible to keep an atmosphere practically free from oxygen in the vicinity of the torch, which therefore does not pollute the molten metal. It also prevents rays from the arc dazzling personnel working on the installation. In addition, it is imperative that the torch act on the bare molten metal, which must therefore not be covered with the thermal insulation powder which it is normal to spread over its surface to protect it from atmospheric reoxidation and to prevent radiation from it.
The refractory materials lining the tundish receive a significant portion of the radiation of the arc emitted by the torch, and because of this their surface is heated to very high temperatures, which can exceed 1800.degree. C. when the torch is used at high power. At these temperatures, magnesium or alumina, which are the materials usually used, reach their melting point and the linings quickly deteriorate. In addition, the refractory material turning to liquid tends to flow over the surface of the metal bath, where it forms an insulating crust which impedes heat transfer between the plasma and the metal, and can even turn off the arc (in the case of a transferred arc plasma torch). It is therefore essential to find an operating point of the torch which achieves a compromise between adequate heating of the metal and tolerable deterioration of the refractory materials, which is to the detriment of the heating efficiency that the torch could theoretically offer.
It is possible to consider making the lining of the tundish in a refractory material with a melting temperature still higher than the conventional materials, for example silicon carbide or a ceramic. But since the lining of the tundish must be completely renewed between each casting or between each sequence, this would considerably increase the cost of operating the installation and would largely cancel out the economic advantages provided by the torch.
In addition, any improvement in the geometry of the tundish that would increase the thermal efficiency of the heating arc would of course be desirable.